An inventory for evaluating hazard and risk assessment of cut slopes in weak rocks along highways

Abstract

Several cases of instability of cut slopes along major highways in Jordan were reviewed in this study, in some detail. Emphasis was placed on the Amman-Irbid highway with the aim of establishing a wider data base of case studies and examining all possible mechanisms and factors influencing stability.

The study showed that major cut slope failures were caused by the presence of weak cohesive layers (mainly clayey marl) interbedded within other, mostly stronger, formations, in addition to the steep cutting angles and unfavourable dip of strata combined with relatively high piezometric surface brought about by poor surface and subsurface drainage.

The study also showed that sections II and III of Amman-Irbid highway suffer from an alarmingly high level of instability in cut slopes with more than 50 % of the entire route length was found to have high level of instability. No remedial measures were carried out so far to improve the overall situation of critical slopes. The amount of excavations required to render the cut slopes stable by satisfying acceptable standards of safety, was determined to be approximately 2.8 million cubic meters with an estimated cost of 10 million JD’s.

To investigate the influence of various parameters on slope stability, several analyses were performed in addition to back analyses to determine shear strength parameters. Data were obtained from previous in vestigations by local and international firms, screened and modified where needed, before being used in stability analyses. Considerable difference between back analysis and test result values for shear strength parameters were observed in many cases.

A classification system was developed for factors influencing landslides, bases on the case studies considered; a stability grading scheme was also introduced; the system was applied to several existing cases along Amman-Irbid highway, the resulting assessments were in agreement with the observed stability conditions.